The invention relates to drying of articles in a process whereby articles are placed in a drying chamber and a heated drying medium is propelled through and among the articles. The invention more particularly relates to such a process where articles continuously, or at intervals, enter a drying chamber in an undried form and exit the drying chamber in a dried form. The invention relates still more particularly to the drying or preheating of plastics or plastic resins in particulate form in such a process.
Plastics and plastic resins used in various production processes are generally manufactured in the form of small particles known as "pellets." When such particles are hygroscopic in nature, they absorb moisture readily and, as a result, generally have a moisture content that is too high for use in many plastic production processes. Accordingly, such particles must be dried prior to being used in subsequent production processes. Moreover, even if the particles are not hygroscopic in nature, they often must be "dried" so that they are pre-heated before they are used in subsequent production process. Conventionally, the drying process involves blowing a stream of heated air through a quantity of the plastic particles that are held in a hopper or other drying chamber. It is also possible to have the particles continuously enter the top of the drying chamber and exit at the bottom of chamber. In such a continuous drying process, the flow rate of the particles generally is adjusted so that the particles, upon exiting the drying chamber, have been exposed to a stream of heated air for a length of time that is adequate to "dry" them, i.e., lower their moisture content or pre-heat them to the desired level.
It has developed that, in such continuous feed processing and drying of plastic pellets and other types of particles, there frequently are stoppages or reductions in the throughput of the particles into and out of the drying chamber. Such stoppages or reductions in throughput may occur for any number of reasons, including routine changing of molds or other "downstream" equipment in various plastic production operations. Regardless of the reasons for such stoppages or reductions in throughput, when they occur, all of the particles in the drying chamber reach an optimum dried condition in a short time period. When the optimum dried condition is reached, no further useful drying occurs and energy is wasted by continuing to heat the air or other drying medium that enters the drying chamber and is blown around and through the particles. In the event heating of the air continues, particles such as plastic pellets may become overheated and begin to degrade or congeal into a solid mass that is unusable in subsequent production processes, with the result that the drying chamber must be dismantled and cleaned and the degraded or congealed particles discarded. Moreover, merely turning off the flow of heated air does not satisfactorily solve this problem because the particles simply will cool off and reabsorb moisture from the ambient air, thereby requiring that additional energy be expended to re-dry them when the flow of heated air through the drying chamber is resumed.
Certain non-continuous or "batch" processes for drying or pre-heating various types of articles are known in the art. In such non-continuous processes, the moisture content of the articles may be estimated based on the behavior of the temperature of the drying medium exiting the drying chamber or by comparison of the temperature of the drying medium before it enters the drying chamber to the temperature of the drying medium after it exits the drying chamber. Depending upon the desired reduction in moisture content or desired level of pre-heating, such non-continuous processes can be "adjusted" by increasing or decreasing the drying time of the particles, i.e.. by allowing a particular batch of articles to remain in the drying chamber for a greater or lesser time period. Thus, for example, in Robinson, U.S. Pat. No. 4,777,604, there is disclosed a method of drying wood products which involves measuring the temperature of a drying medium before and after it contacts the wood products, calculating a difference in these temperatures and, based on this difference, adjusting either the drying time or the temperature of the drying medium before contact with the wood products.
In many continuous feed drying operations, however, adjusting the drying time is not feasible because the length of time the particles are retained in the drying chamber is dictated and controlled by the material feed requirements of "downstream" production processes. Moreover, in present continuous feed processes, it is difficult to control the drying operation by adjusting the temperature of the incoming drying medium, particularly in view of the overriding requirement in most processes that the particles continuously exiting the drying chamber must be thoroughly dried. As a result, during normal operation of known continuous drying processes, the temperature of the drying medium entering the drying chamber generally is maintained at a constant level. This, however, can be wasteful of energy and often results in overdrying of the particles, particularly in those situations where there is a stoppage or reduction in the throughput of the particles being dried, as described above.